The present invention relates generally to an organic electroluminescent device constructed by providing at least an emitting layer and a carrier transport layer using an organic material between a hole injection electrode and an electron injection electrode, and more particularly, to an organic electroluminescent device capable of performing stable luminance for a long time by using an organic material composed of a stable chelate compound as the organic material in the emitting layer and the carrier transport layer to prevent the organic material from being crystallized in the emitting layer and the carrier transport layer.
In recent years, the needs of flat panel display devices the consumption of electric power and the size of which are smaller than those of a CRT (Cathode-Ray Tube) which has been heretofore generally employed have been increased as information equipments are diversified, for example. An electroluminescent device has been paid attention to as one of the flat panel display devices.
The electroluminescent device is roughly divided into an inorganic electroluminescent device and an organic electroluminescent device depending on a used material.
The inorganic electroluminescent device is so adapted that a high electric field is generally applied on a luminescent portion, and electrons are accelerated within the high electric field to collide with a luminescence center, whereby the luminescence center is excited to emit light. On the other hand, the organic electroluminescent device is so adapted that electrons and holes are respectively injected into a luminescent portion from an electron injection electrode and a hole injection electrode, the electrons and the holes thus injected are recombined with each other in a luminescence center to bring an organic molecule into its excited state, and the organic molecule emits fluorescence when it is returned from the excited state to its ground state.
In the case of the inorganic electroluminescent device, a high voltage of 100 to 200 volts is required as its driving voltage because the high electric field is exerted as described above. On the other hand, the organic electroluminescent derive can be driven at a low voltage of approximately 5 to 20 volts.
In the case of the organic electroluminescent device, a luminescent device emitting light in a suitable color can be obtained by selecting a fluorescent material that is a luminescent material. It is expected that the organic electroluminescent device can be also utilized as a multi-color or full-color display device, for example. Further, it is considered that the organic electroluminescent device is utilized as a back light of a liquid crystal display device or the like because it can emit light at a low voltage.
In recent years, various studies have been conducted on such an organic electroluminescent device.
In such an organic electroluminescent device, an emitting layer and a carrier transport layer which is constituted by a hole transport layer for transporting holes to the emitting layer and an electron transport layer for transporting electrons thereto are generally provided between a hole injection electrode and an electron injection electrode. Specifically, used as the structure thereof are a three-layer structure referred to as a DH structure obtained by laminating a hole transport layer, an emitting layer and an electron transport layer between a hole injection electrode and an electron injection electrode, a two-layer structure referred to as an SH-A structure obtained by laminating a hole transport layer and an emitting layer abundant in electron transporting properties between a hole injection electrode and an electron injection electrode, and a two-layer structure referred to as an SH-B structure obtained by laminating an emitting layer abundant in hole transporting properties and an electron transport layer between a hole injection electrode and an electron injection electrode.
In forming the carrier transport layer and the emitting layer in the organic electroluminescent device, various types of organic materials have been conventionally used, to form uniform layers composed of the organic material generally by vacuum evaporation or the like.
However, the organic material generally used for the emitting layer and the carrier transport layer has not conventionally showed sufficient stability. The organic material is gradually crystallized with the elapse of time, so that crystals are deposited on the emitting layer and the carrier transport layer. Consequently, there are some problems. For example, the organic electroluminescent device is short-circuited, for example, so that the overall organic electroluminescent device cannot uniformly emit light.
Conventionally, a chelate compound composed of a plurality of ligands coordinated to one metal ion, for example, tris(8-quinolinol)aluminum (hereinafter referred to as Alq3) indicated by the following chemical formula 1 has been widely utilized as the organic material in the emitting layer and the carrier transport layer. 
However, the chelate compound heretofore widely utilized does not necessarily have sufficient properties. In the case of the above-mentioned Alq3, for example, its luminescent peak wavelength is as large as 520 nm to 530 nm, and its excitation energy is small. When the Alq3 is used as a host material in the emitting layer, there are some problems. For example, a dopant such as perylene having large excitation energy cannot be caused to emit light.
A conventional organic electroluminescent device is so constructed that a hole injection electrode composed of an indium-tin oxide (hereinafter referred to as ITO) having a large work function is provided on a transparent glass substrate, to take out light emitted in the emitting layer outward through the hole injection electrode and the glass substrate.
Since the refractive indices of the emitting layer and the ITO are generally 1.5 to 1.7, and the refractive index of the glass substrate is also generally about 1.5, while the refractive index of air is 1.0, however, a part of the light emitted in the emitting layer is reflected on the interface of the glass substrate and the air, to be confined in the organic electroluminescent device. Therefore, the luminance of the light irradiated outward is reduced.
Furthermore, in the conventional organic electroluminescent device, the electron injection electrode is generally formed of a metal having a small work function in order that electrons are efficiently injected into the organic electroluminescent device from the electron injection electrode.
When the electron injection electrode is formed of a metal having a small work function, however, the electron injection electrode is easily oxidized upon reacting with oxygen and water in the air. When the electron injection electrode is thus oxidized, properties in the electron injection electrode are degraded, so that there are some problems. For example, the luminance is reduced, and dark spots (non-luminescent portions) grow.
An object of the present invention is to solve the above-mentioned various problems in an organic electroluminescent device having at least an emitting layer and a carrier transport layer using an organic material between a hole injection electrode and an electron injection electrode.
That is, an object of the present invention is to prevent, in the above-mentioned organic electroluminescent device, an organic material used for an emitting layer and a carrier transport layer from being gradually crystallized with the elapse of time as in the conventional example, to prevent short, for example, of the organic electroluminescent device.
An object in the organic electroluminescent device according to the present invention is to make it possible to perform stable luminance for a long time by using an organic material composed of a stable chelate compound for the emitting layer and the carrier transport layer.
An object in the organic electroluminescent device according to the present invention is to make it possible to sufficiently cause, even in a case where an organic material composed of a new chelate compound is used for the emitting layer and the carrier transport layer, and a dopant such as perylene having large excitation energy is used, the dopant to emit light.
An object in the organic electroluminescent device according to the present invention is to make it possible to efficiently emit light emitted in the emitting layer outward through a glass substrate.
Furthermore, an object in the organic electroluminescent device according to the present invention is to make it possible to perform stable luminance by preventing an electron injection electrode from being oxidized.
In an organic electroluminescent device constructed by providing at least an emitting layer and a carrier transport layer using an organic material between a hole injection electrode and an electron injection electrode, a first organic electroluminescent device according to the present invention is characterized in that at least one of the emitting layer and the carrier transport layer contains a chelate compound of a chromone derivative with zinc or aluminum.
When a chelate compound of a chromone derivative with zinc or aluminum is used as the organic material in the emitting layer and the carrier transport layer as in the first organic electroluminescent device, the chelate compound is stable and is difficult to crystallize, thereby preventing crystals from being deposited on the emitting layer and the carrier transport layer, to prevent short, for example, of the first organic electroluminescent device. Therefore, stable luminance is performed for a long time.
In an organic electroluminescent device constructed by providing at least a carrier transport layer and an emitting layer using an organic material between a hole injection electrode and an electron injection electrode, a second organic electroluminescent device according to the present invention is characterized in that at least one of the emitting layer and the carrier transport layer contains a chelate compound of a 3-hydroxyflavone derivative with a metal.
When a chelate compound of a 3-hydroxyflavone derivative with a metal is used as the organic material in the emitting layer and the carrier transport layer as in the second organic electroluminescent device, the chelate compound is stable and is difficult to crystallize, thereby preventing crystals from being deposited on the emitting layer and the carrier transport layer, to prevent short, for example, of the second organic electroluminescent device. Therefore, stable luminance is performed for a long time.
In an organic electroluminescent device constructed by providing at least an emitting layer and a carrier transport layer using an organic material between a hole injection electrode and an electron injection electrode, a third organic electroluminescent device according to the present invention is characterized in that at least one of the emitting layer and the carrier transport layer contains a chelate compound composed of an 8-quinolinol derivative dimer coordinated to a metal.
In a case where a chelate compound composed of an 8-quinolinol derivative dimer coordinated to a metal is used as in the third organic electroluminescent device according to the present invention, when the chelate compound is heated in a vacuum state, the chelate compound sublimates, so that an evaporation film is easily formed. Therefore, the emitting layer and the carrier transport layer are simply formed. Further, even when the emitting layer and the carrier transport layer are left under high temperatures after the evaporation film is thus formed, crystals are prevented from being deposited on the emitting layer and the carrier transport layer. Therefore, stable luminance can be performed for a long time.
In an organic electroluminescent device constructed by providing at least an emitting layer and a carrier transport layer using an organic material between a hole injection electrode and an electron injection electrode, a fourth organic electroluminescent device according to the present invention is characterized in that at least one of the emitting layer and the carrier transport layer contains a chelate compound composed of two 8-quinolinol derivatives coordinated to a metal and one halogen bonded thereto.
Three-dimensional distortion of the chelate compound used in the fourth organic electroluminescent device according to the present invention is larger than that of the chelate compound conventionally generally used. When the chelate compound is used for the emitting layer, therefore, its luminescent peak wavelength is shifted toward the shorter wavelength, so that its excitation energy is increased.
When the chelate compound is used as a luminescent material in the emitting layer, therefore, light having a short wavelength is emitted. On the other hand, when the chelate compound is used as a host material in the emitting layer, a dopant having large excitation energy can be caused to emit light. Further, when the chelate compound is used as the host material, and a particular dopant is used, the chelate compound and the dopant form an exciplex. Consequently, the half value width of a luminescent spectrum is increased, so that light different from the light emitted by the dopant and the chelate compound is emitted.
In the present invention, a reflection preventing film for reducing the reflectance is formed on a surface, from which light is emitted, of a glass substrate for emitting light emitted in the emitting layer in each of the above-mentioned first to fourth organic electroluminescent devices.
When the reflection preventing film for reducing the reflectance is formed on the surface, from which light is emitted, of the glass substrate as in the organic electroluminescent device, the light emitted in the emitting layer is efficiently emitted through the glass substrate.
In the present invention, an oxidation preventing protective film is formed on the surface of the electron injection electrode in each of the above-mentioned first to fourth organic electroluminescent devices by ECR (Electron Cyclotron Resonance) plasma CVD (Chemical Vapor Deposition).
When the above-mentioned oxidation preventing protective film is formed on the surface of the electron injection electrode as in the organic electroluminescent device, the electron injection electrode is prevented from being oxidized, thereby preventing the luminance from being reduced and preventing dark spots from growing. As described above, according to the ECR plasma CVD, an oxidation preventing protective film can be formed under low temperatures, and the oxidation preventing protective film can be formed at a high speed of not less than 100 nm/min. Therefore, the oxidation preventing protective film can be efficiently formed without adversely affecting the organic material in the organic electroluminescent device.